1. Field of the Invention
The present invention relates to a method of preparing a pyrrolotriazin-4-one compound and, more particularly, to a method of producing a pyrrolotriazin-4-one compound which includes a step of addition reaction between a thioisocyanate compound and a 2-aminopyrrole derivative and a step of ring-closing reaction of a resulting adduct. The present invention also relates to a method of preparing an isothiocyanatoformic acid ester derivative, which is preferably used in the preparation of pyrrolotriazinone compounds.
2. Description of the Related Art
It has been known that compounds having a pyrrolotriazin-4-one skeleton are useful for use in photosensitive materials, heat-sensitive materials, color developing components in photographic materials, and precursors of biologically active substances. Various derivatives have been synthesized.
Isothiocyanatoformic acid ester derivatives have been used as reactants in the synthesis of pyrrolotriazinone compounds. It has been known that isothiocyanatoformic acid ester derivatives can be synthesized by a procedure described in J. C. S. Perkin I, page 2644 (1973). A preparation scheme of an isothiocyanatoformic acid ester derivative as described in the reference is shown below. 
In the conventional preparation method described above, a hydroxy derivative represented by the general formula (2xe2x80x2) is added to ethyl isothiocyanatoformate that has been formed by reacting potassium isothiocyanate with ethyl chloroformate. However, a portion of the ethyl isothiocyanatoformate decomposes before reacting with the hydroxy derivative represented by the general formula (2xe2x80x2), because reactivity between potassium isothiocyanate and ethyl chioroformate is high and the ethyl isothiocyanatoformate that is formed is unstable. As a result, there arises a problem that unreacted hydroxy derivative remains, thereby lowering purity and yield of an intermediate represented by the general formula (4xe2x80x2).
The intermediate represented by the general formula (4xe2x80x2) and the isothiocyanatoformic acid ester derivative represented by the general formula (Axe2x80x2) (in which Rxe2x80x2 represents a methyl group or an ethyl group) must be purified by crystallizationat a low temperature, that is, lower than xe2x88x9250xc2x0 C. because of low melting points thereof. Therefore, there arises a problem that it is difficult to industrially produce the intermediate and the isothiocyanatoformic acid ester derivative at high purity and high yield because of complexity of operations.
There is also a problem that, in cases where the number of carbon atoms of Rxe2x80x2 in the intermediate represented by the general formula (4xe2x80x2) and the isothiocyanatoformic acid ester derivative represented by the general formula (Axe2x80x2) is 3 or more, the intermediate and the derivative are often in the form of oils. Therefore, it is difficult to industrially produce the same at high purity and high yield in this respect also.
Also, problems arise because the isothiocyanatoformic acid ester derivative represented by the general formula (Axe2x80x2) prepared by the conventional preparation method described above has low purity, ranging from 50 to 60%, and tar components derived from by-products in the preparation of the isothiocyanatoformic acid ester derivative are formed when pyrrolotriazin-4-one is synthesized using the isothiocyanatoformic acid ester derivative as a reactant without purifying. Consequently, operation properties and the yield of pyrrolotriazin-4-one are lowered.
An object of the present invention is to provide a novel method of preparing a pyrrolotriazin-4-one compound, which method is capable of synthesizing a-compound having a pyrrolotriazin-4-one skeleton at high yield in a simple operation. Another object of the present invention is to provide a novel isothiocyanatoformic acid ester derivative, which can be used preferably as a reactant for synthesis of a pyrrolotriazinone compound, and to provide a method of preparing the isothiocyanatoformic acid ester derivative at high purity and high yield.
A means for solving the problems described above is as follows.
A method of preparing a pyrrolotriazin-4-one represented by the following general formula (4). The method includes:
an addition step of reacting an aminopyrrole derivative represented by the following general formula (1) with a reactant represented by the following general formula (2) to form an adduct represented by the following general formula (3), and a cyclization step of cyclizing the adduct represented by the following general formula (3) to form the pyrrolotriazin-4-one represented by the following general formula (4). 
In the formulas; R1 represents a hydrogen atom, an alkyl group, an aryl group, or a group capable of withdrawing. R2 and R3 each independently represents a hydrogen atom, an alkyl group, an aryl group, a cyano group, a substituted sulfonyl group, a substituted carbonyl group, or a halogen atom. R4 and R6 each independently represents an alkyl group or an aryl group. R5 represents an alkyl group, an aryl group, or a heterocyclic group.
Also provided is a method of preparing an isothiocyanatoformic acid ester derivative represented by the following general formula (2). The method includes a step of adding a chloroformic acid derivative represented by the following general formula (7) to an isothiocyanic acid salt represented by the following general formula (5) and a hydroxy derivative represented by the following general formula (6) to prepare an intermediate represented by the following general formula (8):
ZNCSxe2x80x83xe2x80x83General formula (5)
R6OHxe2x80x83xe2x80x83General formula (6)

Z represents a sodium atom or a potassium atom in the general formula (5). R4 and R6 in the general formulas (6), (7) and (8)are as defined for the general formula (2).
Further provided is a method of preparing the isothiocyanatoformic acid ester derivative represented by the general formula (2). This method includes a step of preparing an intermediate represented by the following general formula (10) from an intermediate represented by the following general formula (8) and a compound represented by the following general formula (9). 
M represents an alkali metal atom, an alkali earth metal atom, an aluminum atom, or a magnesium atom in the general formula (9) R4 and R6 in general formula (10) are as defined in the general formula (2).
Another method of preparing the isothiocyanatoformic acid ester derivative represented by the general formula (2) includes a step of reacting an intermediate represented by the following general formula (10) with an alkylating agent represented by the following general formula (12) to prepare the isothiocyanatoformic acid ester derivative represented by the general formula (2). xe2x80x83R5Xxe2x80x83xe2x80x83General formula (11)
(R5O)2SO2xe2x80x83xe2x80x83General formula (12)
X represents a halogen atom or SO3Ar in the general formula (11). Ar represents a substituted or non-substituted aryl group. R5 in the general formulas (11) and (12) is the same as that in the general formula (2).